To build an optical signal distribution network within a semiconductor substrate, one needs to be able to make good optical waveguides to distribute the optical signals and one needs to be able to fabricate elements that get the optical signals into and out of the waveguides to interface with other circuitry. Extracting the optical signals can be accomplished in at least two ways. Either the optical signal itself is extracted out of the waveguide and delivered to other circuitry that can convert it to the required form. Or the optical can be converted into electrical form in the waveguide and the electrical signal delivered to other the circuitry. Extracting the optical signal as an optical signal involves the use of mirrors within the waveguides or elements that function like mirrors. The scientific literature has an increasing number of examples of technologies that can be used to construct such mirrors. Extracting the optical signal as an electrical signal involves the use of detector within the waveguide, i.e., circuit elements that convert the optical signal to an electrical form. The scientific literature also has an increasing number of examples of detector designs that can be used to accomplish this.
The challenge in finding the combination of elements that produces an acceptable optical distribution network becomes greater, however, when one limits the frame of reference to particular optical signal distribution network designs and to the financial reality that any such designs should be easy to fabricate and financially economical.
The combination of silicon and SiGe has attracted attention as useful combination of materials from which one might be able to easily and economically fabricate optical signal distribution networks. With SiGe it is possible to fabricate waveguides in the silicon substrates. The index of refraction of SiGe is slightly higher than that of silicon. For example, SiGe with 5% Ge has a index of refraction of about 3.52 at an optical wavelength of about 1300 nm while crystalline silicon has an index of refraction that is less than that, e.g. about 3.50. So, if a SiGe core is formed in a silicon substrate, the difference in the indices of refraction is sufficient to enable the SiGe core to contain an optical signal through internal reflections. Moreover, this particular combination of materials lends itself to the use of conventional semiconductor fabrication technologies to fabricate the optical circuitry.
Of course, for such a system to work as an optical signal distribution network, the optical signal must be at a wavelength at which the Si and SiGe are transparent. Since the bandgap of these materials is about 1.12 eV, they appear transparent to the commonly used optical wavelengths of greater than about 1100 nm. But, the transparency of these materials to optical signals having those wavelengths brings with it another problem. These materials are generally not suitable for building detectors that can convert the optical signals to electrical form. To be a good detector, the materials must be able to absorb the light. That is, the optical signal must be capable of generating electron transitions from the valence band to the conduction band within the detector to produce an electrical output signal. But the wavelengths of greater than about 1100 nm are too long to produce electron transitions in silicon. For example, at a wavelength of 1300 nm, the corresponding photon energy is about 0.95 eV, which is well below the bandgap of silicon or SiGe and consequently well below the amount necessary to cause transitions from the valence band into the conductor band.
One class of detectors that has attracted some interest is the class of SiGe super lattice detectors. These detectors are made up of alternating thin layers of Si and SiGe. Because the lattice constant of these materials is not the same, when the two layers are grown on top of each other the lattice mismatch causes a strain in the SiGe layer. If the Si and SiGe layers are sufficiently thin (e.g. on the order of about 6 nm), and if the process temperatures to which the structure is exposed are sufficiently low (e.g. below about 800° C.), then the induced strain will be permanent. The induced strain reduces the bandgap of the SiGe material. As the percentage of Ge in the SiGe increases, the mismatch becomes greater, the induced strain increases and the bandgap decreases further.
FIG. 1 illustrates how the percentage of Ge impacts the bandgap in the super lattice structures. If the induced strain is maintained in the SiGe, as the percentage of Ge increases, the bandgap decreases along the lower curve. At some point the percentage of Ge will be enough to reduce the bandgap sufficiently so that it can serve as a detector for light having wavelengths of about 1200 nm (about 0.9 eV). However, if the lattice is allowed to relax thereby relieving the strain, the affect of increasing amounts of Ge on the bandgap will be less dramatic as indicated by the upper curve and it will not be possible fabricate an effective detector for that wavelength.